A calculating method for inversing body wave velocities and three-dimensional structure of interfaces by using reflected wave travel time is presented.A fast 3-D ray tracing algorithm by Chander and a damped least-squares technique are used to solve forward and inversion problems respectively.The result shows that the solutions of inversion are close quite to the true model.A 3-D Moho interface structure in Tangshan earthquake region is constructed using P_mP reflected wave data.The faults of Moho interface in Tangshan area and its vicinity are relative to the Tangshan main earthquakes and a series of aftershocks in 1976.